Grammar is the servant of meaning.
If the illness, regardless of manifestations, deserves examination by a physician, then "warrants" is the proper word. So cancer warrants attention by a doctor regardless of signs or symptoms.
If the manifestations, regardless of cause, deserve treatment by a physician, then "warrant" is the proper word. So a prolonged hemorrhage and simultaneous high fever warrant attention by a doctor regardless of etiology.
In the example sentence, what seems to be of importance are plural manifestations, and so "warrant" is the grammatical choice given what seems to be the intended meaning.
By the way, if you are trying to be precise, the manifestations mentioned in your example are signs rather than symptoms.